In Logic - and most daws - each track or channel uses one core. You can't really distribute an AU's load across multiple cores, nor can you split individual plugins on the same channel, to multiple cores. Even Kontakt, which in standalone mode can be multicore aware, does not distribute stuff when used as an AU/VST.
The problem with the 2.26 is that it simply is too slow - per core - to run the most thirsty plugins these days. You can run hundreds of low overhead plugins that distribute nicely over the 8 cores - but the actual core speed isn't that great, unfortunately.
I have the same issue - i have a 2008 Mac Pro 2.8ghz octo and i can run out of cpu on one core while the other 7 do very little - logic uses the last core to perform in 'live mode' when you select an instrument track. I run a native rig (no hardware monitoring) and run at a 64 sample buffer (SSL MadiXtreme PCIe interface) and running Kontakt with something big ilke Abbey Road Drums, with a 64 buffer and lots of polyphony... it can get nasty. And routing multiple outs from Kontakt/any AU to multiple auxs and throwing plugins on them (i.e a kontakt drum kit routed out in logic with kick/snare/hihat/room all on their own aux).... forget about it. For whatever reason, Logic does all that on ONE CORE... ridiculous.
Diva is brand new and uses a lot of cpu. In order for it to sound so good, Urs has basically said 'yeah... it's cpu hungry, sorry', especially with pad sounds with high polyphony...
Workarounds/solutions - assuming you aren't going to just go out and buy a new Mac Pro:
- Increase your buffer size. If you're just at the mixing stage, throw it all the way up 1024! When composing, you might be OK living with 256 or 128, see how they feel - what are you running now?
- Run in 64-bit mode - it gave me an extra boost in cpu headroom of around 30% in some cases
- Reduce oversampling in Diva, increase it when bouncing (offline)
- Turn off convolution reverbs in Kontakt 5 - they are power hungry. I'm sure Kontakt 5 is worse than 4 for cpu usage... and lower the polyphony
- Make sure that when you're playing back the song in Logic, the Diva/Kontakt/au instrument channel is not selected in the arrange, as logic will record/live arm the track, which will send the cpu usage on the last core to go bananas
- If you have enough RAM, use multiple instances Kontakt with the same patch loaded. This allows each instance of Kontakt to do some of the polyphony, and it'll also distribute these across multiple loads.
- Do not use multiple patches in one Kontakt instance. Again, use one patch per instance of Kontakt. This is a manual way of distributing Kontakt instruments across cores
Other options for you are: upgrade the cpu in your mac pro (whole other subject/thread, but you can get 2010 cpu's and drop them in your machine, if you have the know how), or just buy a newer mac pro like the 6 core 3.33ghz, or whatever the new machines will be. The 2010 are basically over twice as fast, per core.
Hope this helps!
Eddie